The present invention relates to a rotary vacuum kiln and method for the treatment of solid particulate material under conditions of high temperature and under a high vacuum.
Solid particulate material must, at times, be treated under a vacuum at high temperatures in order to provide a desired product. In the manufacture of tantalum powders, for example, for use in capacitors, at one or more steps in processing, the powder is heat-treated in a vacuum furnace. Such treatment may be used to drive off residual impurities and to provide a flowable powder. A present processing system involves placement of a stack of trays containing tantalum powder into a vacuum furnace and heating the entire tray assembly. After a comparatively short heat treatment, in such a batchwise treatment, the entire tray assembly is cooled and a small amount of air is admitted until a layer of tantalum oxide has formed on the powder particle surfaces to prevent pyrophoric combustion of the powder upon subsequent exposure to air. Such a treatment is time-and energy-consuming and requires expensive equipment. Also, the fixed bed geometry of the treatment results in material near the outside of the bed being heated sooner and hotter than the material in the middle of the bed or tray stack. Heat transfer is also slow. In addition, since the material on the outside of the bed is heated more than that on the inside, uneven sintering can occur. A non-uniform product can result with various portions of the charge having different physical properties from others. If the material on the inside is not sufficiently sintered, the resultant product is fragile and a large proportion of this material turns to a dust during subsequent handling of the product. Such dust or fines must be recycled for reprocessing.
It is an object of the present invention to provide an apparatus for high temperature treatment of solid particulate material, while under a vacuum, by the use of a rotating kiln that will provide a more heat-treated uniform product.
It is another object of the present invention to provide a method for the continuous high temperature treatment of solid particulate material, such as tantalum powder, while under a vacuum, using a rotating kiln so as to provide a more uniform heat treated product.